The impedance measuring vectors or paths provided by some modern pacemakers and implantable cardio defibrillators are quite extensive. Many pacemakers currently measure impedance to measure minute ventilation as a physiological indicator of activity. The minute ventilation value obtained in this way can be used to set the pacing rate in a physiological adaptive pacemaker. The impedance changes over time over a particular vector can have many contributing factors, some major and some minor, so that multiple factors contribute to impedance signals measured by the device. A nonexclusive list of such contributing factors in which changes in the factors over time can cause changes in the measured impedance over time across a vector include, for example, changes in lung resistivity, changes in blood resistivity, changes in heart muscle resistivity, changes in skeletal muscle resistivity, changes in heart volume, and changes in lung volume. Measuring changes in impedance or resistivity in a certain contributing factor can be problematic, since such changes tend to be relatively accurately detectable across one vector while being less susceptible to accurate detection across another vector. Some vectors are highly sensitive or susceptible to changes in certain of the contributing factors, while being less sensitive or susceptible to impedance changes in other contributing factors.
What is needed is a method and apparatus that more accurately differentiates between the multiple sources of and/or physiological factors that contribute to changes in impedance measures over time.